U.S. patent number 4,549,155 [Application Number 06/420,433] was granted by the patent office on 1985-10-22 for permanent magnet multipole with adjustable strength.
This patent grant is currently assigned to The United States of America as represented by the United States. Invention is credited to Klaus Halbach.
United States Patent |
4,549,155 |
Halbach |
October 22, 1985 |
Permanent magnet multipole with adjustable strength
Abstract
Two or more magnetically soft pole pieces are symmetrically
positioned along a longitudinal axis to provide a magnetic field
within a space defined by the pole pieces. Two or more permanent
magnets are mounted to an external magnetically-soft cylindrical
sleeve which rotates to bring the permanent magnets into closer
coupling with the pole pieces and thereby adjustably control the
field strength of the magnetic field produced in the space defined
by the pole pieces. The permanent magnets are preferably formed of
rare earth cobalt (REC) material which has a high remanent magnetic
field and a strong coercive force. The pole pieces and the
permanent magnets have corresponding cylindrical surfaces which are
positionable with respect to each other to vary the coupling
therebetween. Auxiliary permanent magnets are provided between the
pole pieces to provide additional magnetic flux to the magnetic
field without saturating the pole pieces.
Inventors: |
Halbach; Klaus (Berkeley,
CA) |
Assignee: |
The United States of America as
represented by the United States (Washington, DC)
|
Family
ID: |
23666451 |
Appl.
No.: |
06/420,433 |
Filed: |
September 20, 1982 |
Current U.S.
Class: |
335/212; 335/304;
335/306 |
Current CPC
Class: |
H01F
7/0278 (20130101) |
Current International
Class: |
H01F
7/02 (20060101); H01F 001/00 () |
Field of
Search: |
;335/302,306,301,304,210,212 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Harris; George
Attorney, Agent or Firm: King; Patrick T. Clouse, Jr.;
Clifton E. Hightower; Judson R.
Government Interests
The U.S. Government has rights 1n this invention pursuant to
Contract No. DE-AC03-76SF00098 (formerly Contract No.
W-7405-ENG-48) with the U.S. Department of Energy.
Claims
What is claimed is:
1. A multipole permanent magnet structure for guiding, focusing and
turning charged particle beams, said structure having an adjustable
field strength and a substantially constant magnetic field
distribution, comprising:
a first pole piece and a second pole piece, each formed of
magnetically-soft material, each pole piece having a pole tip, said
pole pieces being spaced-apart to permit a magnetic field to be
established between the pole tips, said pole pieces being arranged
about a longitudinal axis to provide a cylindrical multipole
structure having a central space formed between the pole tips and
extending along the longitudinal axis for passage of a charged
particle beam through the space;
first and second permanent magnets having high remanent fields and
strong coercive forces, said permanent magnets being mounted in
close proximity to the rear of said pole pieces and magnetically
coupled thereto to thereby establish a magnetic field between said
pole tips;
means for moving the permanent magnets with respect to the pole
pieces to vary the coupling between the pole pieces and the
permanent magnets so that the flux denisity of the magnetic field
between the pole tips is correspondingly varied, while the magnetic
field distribution between the pole tips is maintained
substantially constant.
2. The magnet structure of claim 1 including a magnetically-soft
sleeve to which the permanent magnets are fixed and which is
rotatable about the rear of the pole pieces.
3. The magnet structure of claim 1 including auxiliary permanent
magnets having high remanent fields and strong coercive forces and
positioned between the pole pieces to provide additional magnetic
flux to the pole pieces and, for strong magnetic fluxes, preventing
saturation of the pole pieces.
4. The magnet structure of claim 1 including a corrector permanent
magnet positioned between the pole pieces such that its magnetic
field opposes and prevents coupling of undesired magnetic fields
from the permanent magnet into the pole pieces.
5. The magnet structure of claim 1 including a plurality of
symmetrically arranged pole pieces and a plurality of permanent
magnets which form a symmetric variable-strength multipole magnet,
said plurality of permanent magnets being greater in number than
said pole pieces.
6. The magnet structure of claim 5 including:
four pole pieces arranged around the longitudinal axis and defining
the space extending along the longitudinal axis, each pole piece
having a cylindrical rear surface,
four permanent magnets having cylindrical surfaces matching the
cylindrical rear surfaces of the pole pieces, said permanent
magnets being movable with respect to the pole pieces; and
a magnetically-soft sleeve providing magnetic coupling between the
four permanent magnets.
7. The magnet structure of claim 6 including four auxiliary
permanent magnets positioned between adjacent pole pieces to
provide additional magnetic flux to the pole pieces.
8. The magnet structure of claim 1 wherein the permanent magnets
are formed of material including rare earth cobalt material.
9. The magnet structure of claim 1 including a plurality of
permanent magnet blocks and a magnetic shield plate positioned at
the end of the permanent magnet structure and coupled to each of
the pole pieces through one of the blocks of permanent magnet
material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to variable field strength magnets and, more
particularly, to multipole variable permanent magnets.
2. Prior Art
A number of techniques are available for producing
variable-strength magnetic fields. Such fields are particularly
useful in charged particle acclerators for bending and focusing of
particle beams. Electromagnets, that is, devices which produce
magnetic fields using electrical currents passing through ordinary
or superconducting windings, have serious limitations for certain
applications. One limitation is the large amounts of expensive
electrical power that these systems consume either for the current
to operate a conventional conductor or for cooling a
superconductor. In addition, conventional electromagnets are
limited to certain minimum volumes because their current densities
are inversely proportional to their linear dimensions, which leads
ultimately to insurmountable cooling problems. The result is that
the currents for these electromagnets must be reduced for smaller
sizes with consequently smaller magnetic fields.
And so it has been found that for many magnet applications it is
often advantageous to use permanent magnets instead of
electromagnets in order to eliminate windings with their consequent
power consumption and to produce strong fields in physically small
spaces. For magnets which are used in small spaces and which
require large pole tip fields, it is very often difficult to
provide enough copper cross-sectional area in the space available.
An area where high-field permanent magnets find particular
application is in the construction of small quadrupole magnets for
guiding, focusing, and turning charged particle beams in linear
accelerators used in atomic physics and medical treatment and
research. A theoretical analysis is presented by J. B. Blewett in
"Design of Quadrupoles and Dipoles Using Permanent Magnet Rings,"
Brookhaven National Laboratory Report No. AADD-89, Aug. 10, 1965.
That report includes equations and analyses for maximizing the
strength of a ring or cylindrical quadrupole permanent magnet using
anisotropic material.
A technique for designing permanent magnet multipole magnets was
disclosed in a paper by the present inventor, K. Halbach, "Design
of Permanent Magnet Multipole Magnets with Oriented Rare Earth
Cobalt Materials," Nuclear Instruments and Methods 169 (1980) pp
1-10. Disclosed therein is a quadrupole design which uses a number
of magnetically anisotropic magnet segments, each having an easy
axis, or axis of magnetic orientation, in a different predetermined
direction. One proposed application of this design combines two
multipole magnets such that one quadrupole is located within the
aperture of the other. For the rare earth cobalt (REC) materials
used, superposition of the individual magnetic fields is possible,
and the fields of each quadrupole add or subtract depending upon
their relative rotational positions. This design suffers from
fringe fields at the ends of the magnet which combine to produce
undesired perturbations in the beam optical properties of the
magnet.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a multipole
permanent magnet having an easily adjustable field strength.
It is another object of the invention to provide an adjustable
multipole permanent magnet which maintains its field distribution
substantially undisturbed as its strength is varied.
It is another object of the invention to provide a magnet having a
variable field strength which does not consume electrical
power.
It is another object of the invention to provide for continous
variation in field strength of a multipole permanent magnet.
In accordance with these and other objects of the invention, a
multipole permanent magnet structure is provided which has an
adjustable field strength. Two or more spaced-apart
magnetically-soft pole pieces are energized by one or more
permanent magnets, which are characterized as having high remanent
fields and strong coercive forces. One preferred group of materials
which has these characteristics are the rare earth cobalt (REC)
materials. In its broadest aspects, means are provided for variably
coupling magnetic flux provided by the one or more permanent
magnets to the pole pieces. This variable coupling is used to
control the field strength of the magnetic field between the pole
pieces while the field distribution of that magnetic field is
maintained substantially constant.
According to one aspect of the invention, the variable coupling for
magnetic flux of the permanent magnets to the pole pieces is
obtained by the pole pieces and the permanent magnet each having
surface areas which move relative to one another and which provide
magnetic coupling therebetween when the surfaces are in close
proximity. Movement of one surface with respect to another places
various portions of the respective surface areas in close proximity
to thereby control the magnetic field strength between the pole
pieces.
In one preferred embodiment of the invention, permanent magnets are
mounted for rotation on a magnetically-soft cylindrical sleeve
which rotates around the pole pieces. Auxiliary permanent magnets
provide additional magnetic flux to the pole pieces and corrector
permanent magnets prevent coupling of undesired fields from the
permanent magnets into the pole pieces.
The method according to the invention includes positioning of the
pole pieces around an axis and exciting the pole pieces with one or
more permanent magnets. Adjustment of the magnetic field strength
on the space between the poles is accomplished by moving the
permanent magnets with respect to the pole pieces to obtain various
degrees of proximity to vary the magnetic coupling
there-between.
One specific preferred embodiment is a symmetric quadrupole in
which four pole pieces are symmetrically arranged around a
longitudinal axis and four permanent magnets are mounted to a
cylindrical sleeve surrounding the pole pieces. Corresponding
cylindrical surfaces are formed on the pole pieces and the
permanent magnets so that, as the sleeve is rotated, variable
magnetic coupling is obtained.
Additional objects, advantages and novel features of the invention
will be set forth in part in the description which follows, and in
part will become apparent to those skilled in the art upon
examination of the following or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawings, which are incorporated and form a part
of the specification, illustrate an embodiment of the invention
and, together with the description, serve to explain the principles
of the invention. In the drawings:
FIG. 1. is B-H curve for a rare earth cobalt (REC) material taken
in the direction parallel to the easy axis thereof;
FIG. 2. is a diagrammatic sectional view of a quadrupole permanent
magnet having a variable field strength in the space provided in
the center thereof;
FIG. 3 is a cross-sectional view of an embodiment of a variable
quadrupole permanent magnet structure according to the invention;
and
FIG. 4 1s sectional view taken along section line 4--4 of FIG.
3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Reference is now made in detail to the present preferred embodiment
of the invention which illustrates the best mode presently
contemplated by the inventor of practicing the method and apparatus
of the invention, a preferred embodiment of which is illustrated in
the accompanying drawings.
As indicated above, for certain application a very important
advantage of a permanent magnet over an electromagnet is that
permanent magnets can be made very small without sacrificing
magnetic field strength. Recall that the current density of an
electromagnet is inversely proportional to the size of the magnet.
Currently available oriented rare earth cobalt (REC) materials
produce magnetic fields that are at least as strong as those
produced by conventional electromagnets of any arbitrary size. In
comparison to other more conventional magnetic materials, REC
materials have relatively simple characteristics which are easy to
understand and to treat analytically. These characteristics have
made REC materials good candidates for improved magnet designs such
as described in this specification.
The process by which REC materials are produced is briefly
described for purposes of understanding its characteristics. A
molten mixture of approximately five parts cobalt to one part of a
rare earth, such as samarium, is rapidly cooled and then crushed
and milled to yield crystalline particles having dimensions on the
order of 5 micrometers. These crystalline particles are highly
anisotropic and have a preferred magnetic polarization direction in
one crystalline direction. A very strong magnetic field is applied
which causes the individual particles to physically rotate until
their magnetically preferred axes are aligned parallel to the
applied magnetic field. Pressure is applied to form manageable
blocks of material and the aligned blocks of material are then
sintered and finally subjected to a very strong magnetic field in a
direction parallel or antiparallel to the previously established
preferred magnetic direction to reestablish full magnetization.
This aligns almost all of the magnetic moments in the direction of
magnetization called the easy axis. The particular characteristic
that makes REC so useful is that this remament magnetic field is
extremely strong and can be changed only by applying a strong
magnetic field in the direction opposite to the field originally
used to magnetize the REC material.
Referring now to the drawings, FIG. 1 shows the B-H curve taken in
the direction of the so-called easy axis for a rare earth cobalt
(REC) material. This curve has several important features. It is
practically a straight line over a wide range of field strengths
and has a slope near unity. The offset of the curve from the
origin, that is the remanent field B.sub.r is typically 0.8 to 0.95
Tesla with the coercive field about 4 to 8 percent less than the
remanent field. This linearity over a wide range of field strengths
and the differential permeability close to unity permits this type
of material to be treated as a vacuum with an imprinted charge or
current density. The consequence of this is that fields produced by
different pieces of REC material superimpose linearly and that this
field can be analytically determined quite easily in the absence of
magnetically soft material, that is, materials which are linear and
which have no hysteresis.
There are several other materials which have properties similar to
REC material, which include resin-bonded REC material and some of
the oriented ferrites, but these have lower remanent fields and
larger permeabilities. These materials can be used to practice the
invention disclosed herein and it is intended that these materials
be generically included with the REC materials to practice the
preferred embodiments of the invention.
Referring now to FIG. 2 of the drawings, a quadrupole version of
the invention is shown in diagrammatic form as a typical radial
section through a cylindrical prism.
A multipole field magnetic field is generically a two-dimensional
field that is dependent on two directional coordinates and that is
independent of the third directional coordinate. The strength of
such a field is proportional to an integer power of r where r is
the shortest distance from the point under consideration to the
axis extending in the third direction. For a quadrupole field, the
field strength is directly proportional to r.
A quadrupole configuration is described as a preferred
configuration of this invention, but it should become readily
apparent that any multipole configuration desired, that is, dipole,
octupole, etc. or any combination thereof to achieve special field
configurations, can be provided and the invention is applicable
thereto.
Four pole pieces 10 of magnetically-soft iron or steel material are
arranged as shown around a central axis 12 extending
perpendicularly to the plane of the figure. The pole pieces
symmetrically extend in directions parallel to the axis 12 and have
similar cross sections at various points along that axis. Each pole
piece has a pole tip portion 14, which for a quadrupole, has a
hyperbolic configuration which is blended into a straight side, as
shown, to provide an optimized field distribution. The rear
surfaces 16 of the pole pieces are shaped as portions of
cylindrical surfaces.
Four permanent magnets 18 formed of a number of bars of suitable
rare earth cobalt (REC) material, or material having similar high
remanent field characteristics, are fixed with a suitable adhesive
material to the inner surface of a cylindrical sleeve 20. The
direction of the magnetic flux provided by each of the permanent
magnets is indicated by an arrow which represents the easy axis of
each magnet. The sleeve 20 is formed of magnetically-soft material
and provides a flux path between the various permanent magnets 18.
The inner surfaces 22 of the permanent magnets 18 are cylindrically
shaped as shown to correspond to the cylindrical shapes of the rear
surfaces 16 of the pole pieces 10. These surfaces 16,22 provide a
means for coupling the magnetic flux of the permanent magnets 18 to
the pole pieces 10. This coupling is variable because, as the
sleeve 20 is rotated, varying amounts of surface areas are placed
in close proximity such that the magnetic flux provided by the
permanent magnets 18 passes through the small air gap therebetween
and is coupled from the permanent magnets 18 to the pole pieces 10.
The pole pieces 10 provide a magnetic path for this flux to the
pole tips 14 which are shaped to distribute the flux in the space
provided between the pole pieces along the axis 12. Thus by
rotating the position of the permanent magnets 18 in the direction
indicated by arrow 25 from the starting position as shown in FIG.
2, the field strength of the field can be adjusted over a range to
a desired value for a particular application without disturbing the
field distribution. This is possible because the permanent magnets
18 are formed of REC material, that is, material with a high
remanent field and a strong coercive force.
FIG. 2 also shows four auxiliary permanent magnet assemblies
composed of a first auxiliary magnet 26 having a rectangular cross
section and a second auxiliary magnet 28 having a trapezoidal cross
section. Both are formed of REC material, and are fixed in position
between the pole pieces 10. The direction of the easy axes are
indicated by the arrows and indicate the direction of the magnetic
fields provided by these magnets. The auxiliary permanent magnets
26,28 provide additional magnetic flux to the respective pole tips
14. This permits strong magnetic fluxes to be available at the pole
tips 14 while preventing saturation of the pole pieces 10.
It should be appreciated that the net magnetic flux supplied to the
pole tip 14 of a particular permanent magnet 10 varies depending on
the rotational position and the polarity of the permanent magnets
18 and depending on the polarity of the fixed auxiliary permanent
magnets 26,30.
Corrector permanent magnets 30 formed from slabs of REC material
are fixed adjacent the pole pieces near the permanent magnets 18.
The corrector permanent magnets 30 are chosen to have thicknesses
and magnetic field strengths and directions which oppose undesired
permanent magnet fields which might enter the sides of the pole
pieces and upset the symmetry of a quadrupole field.
Referring now to FIGS. 3 and 4 of the drawings, a preferred
embodiment of a quadrupole variable-strength permanent magnet is
shown. This preferred embodiment is very similar to that shown in
FIG. 2 with the addition of certain functional details to
facilitate the making and using thereof.
Four magnetically-soft pole pieces 40 are mounted at each end to
two nonmagnetic disc-shaped end plates 42 with a series of pins 44
wedged into corresponding holes in the pole pieces 40 and the end
plates 42. The end plates 42 are adapted to have suitable support
structure attached thereto for mounting the quadrupole magnet in
position, for example, in a charged-particle beam line which sends
particles along a longitudinal axis 46. The quadrupole magnet
serves as part of a magnetic means for focusing the particle
beams.
Each of the pole pieces 40 has a hyperbolically-shaped pole tip 48
positioned along the axis 46 to provide a magnetic field within the
space defined by those symmetrically spaced-apart pole tips. Four
auxiliary permanent magnet assemblies are formed from a series of
REC magnets 50 having rectangular cross sections. The magnets 50
are fixed in position between the pole pieces 40 by a suitable
adhesive material. The auxiliary magnets 50 are formed of REC
material having easy axes as indicated to provide magnetic flux to
the pole tips 48.
As shown in FIG. 4, a series of elongated REC bars 60 having
rectangular cross sections are fixed with a suitable adhesive
material to the interior surface 62 of a magnetically-soft
cylindrical sleeve 64 to form the four permanent magnets. The
interior surfaces of the permanent magnets formed by the bars 60
are located next to a nonmagnetic inner sleeve 66. The ends of the
inner sleeve 66 are fixed within corresponding slots on the inside
walls of a pair of sleeve-mounting flanges 68, which also mount the
ends of the the magnetically-soft cylindrical sleeve 64 for
rotation about the longitudinal axis 46. The inner surfaces of the
flanges 68 engage the outer surfaces of the disk-shaped mounting
plates 42 with the interface therebetween serving as a rotational
bearing for the sleeve 64 and the attached permanent magnets
60.
Corrector permanent magnets 52 formed of slabs of REC material and
oriented as indicated are fixed adjacent and between the pole
pieces 40 near their outer edges and close to the permanent magnet
bars 60. The corrector permanent magnets 52 have magnetic field
strengths which oppose undesired fields from the permanent magnets
which might enter the sides of the pole pieces near their
interfaces with the auxiliary permanent magnets 50. These undesired
fields would upset to some degree the symmetry of the quadrupole
for certain rotational positions of the permanent magnets as the
cylindrical sleeve 64 is rotated in the direction of arrow 70
beginning, for example, from the starting position shown in FIG.
3.
Fixed to each end plate 42 is a magnetically-soft shield plate 71
which is coupled to each of the pole pieces 40 through four blocks
72 of REC material. This shields the ends of quadrupole structure
from stray external fields and confines and shapes the magnetic
field of the quadrupole near its ends.
FIG. 4 shows a means for rotating the cylindrical sleeve 64 which
includes a stepper-motor 74 driving a backlash free worm 76 which
engages a ring gear 78 fixed to the sleeve-mounting flange 68. The
position of the permanent magnets 60 with respect to the pole
pieces is controlled by the stepper motor to thereby obtain a
desired magnetic field strength for the quadrupole.
The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed, and obviously many
modifications and variations are possible in light of the above
teachings. The embodiment was chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto and their equivalents.
* * * * *